Microglial cells are resident innate immune cells in the brain and exhibit dynamic changes in morphology and distribution during gestation. They can regulate diverse programs that are essential for normal brain development and function, and recent studies have shown that in the developing brain microglia regulate key processes including synapse development, axonal path finding, and cortical layer formation. We have shown that microglia regulate cell production through phagocytosis of neural precursor cells. However, it is currently not known how microglial cell lineage(s) correlate with colonization of the brain or the cellular function of microglial cells in the developing brain, particularly in relation to neural precursor cells. The studies proposed in this application are designed to address significant gaps in our knowledge on the trafficking and seeding of microglial cell populations during critical windows of nonhuman primate brain development. These studies will generate new insights into normal development that are critical to our understanding of ontogeny and will directly address the goals of PAS-18-483 in a translational nonhuman primate model through the following Specific Aims: (1) Evaluate the role of yolk sac precursor cells in contributing to the microglial cells that populate the fetal rhesus monkey cerebral cortex; (2) Determine the role of clonal lineage(s) in regional colonization of the cerebral cortex and function of cortical microglia; and (3) Define temporal dynamics that impact the distribution and function of clonal microglia. Together the proposed studies will address fundamental gaps in our understanding of factors that regulate colonization of the cerebral cortex by microglial cells, and the impact microglial cell function has on the developing brain.